A wide assortment of body-implantable medical devices having some level of automaticity are presently known and commercially available. The class of such devices includes cardiac pacemakers, cardiac defibrillators and cardioverters, neural stimulators and many others.
Most body-implantable devices are contained within a hermetically sealed enclosure, in order to protect the operational components of the device from the harsh in-vivo environment, as well as to protect the body from the device. Additionally, many implantable devices, operate in conjunction with one or more electrically conductive leads, adapted to conduct electrical stimulating pulses to sites within the implant patient's body, and to communicate sense signals from those sites back to the implanted device. A connector block assembly of some sort is typically used to establish the electrical connection between the lead(s) and the internal components of the device and at the same time maintain the hermetic integrity of the enclosure. Almost all current Implantable Pulse Generators (IPGs) have a hard plastic connector block mounted on a titanium hermetically sealed housing which holds the pulse generator and other circuitry and sensors. For non-IPG implantable devices which perform other medical functions, other sensors and circuits could be so housed.
Typically it is necessary to provide an automatic, body implantable device with a source of power, e.g., a battery, housed within the hermetic enclosure of the device. Battery longevity is often a critical consideration in the design and implementation of body implantable devices. It is highly impractical to replace the battery of an implanted device, and it is clearly desirable to require replacement of an implanted device--a surgical procedure--as infrequently as possible.
Because battery longevity and battery depletion are of such critical concern in relation to most implantable devices of any function, there have been numerous approaches taken for minimizing power consumption of such devices. For pacemakers, it is well-known, for example, that the pacing pulse energy content should be programmed to a level that is only as high as necessary to ensure that pacing pulses will consistently capture the patient's heart. Unnecessarily high-energy pacing pulses are wasteful of a pacemaker's power supply and hence unnecessarily reduce useful device longevity.
Another known approach to power conservation in implantable devices is to design the devices such that various electronic subsystems of the device can be independently activated (powered up) and deactivated (and powered down) as and when desired. For example, an implantable device's telemetry system need only be activated during programming sessions. This is recognized in U.S. Pat. No. 5,342,408 to de Coriolis et al., entitled "Telemetry System for an Implantable Cardiac Device." Likewise, a rate-responsive pacemaker's activity sensing and minute continuation subsystems need not be activated when the device is programmed to operate in a non-rate-responsive mode.
It should be mentioned that all references cited within this application are deemed incorporated herein by this statement of reference so as to obviate the need for redundant explanations of known art.
In an implantable device whose operation is controlled by a microprocessor, it has been recognized that power can be conserved by partially or fully deactivating the microprocessor during intervals where no processing is necessary. Modern microprocessors, which are capable of executing potentially millions of instructions per second, can accomplish the instruction processing necessary to control an implantable device's operation in a very short time relative, for example, to the length of a typical human cardiac cycle. This means that it is not necessary for the processor to be fully active, or to be operating at its fastest possible clock speed, for a substantial portion of each cardiac cycle.
U.S. Pat. No. 4,404,972 to Gordon et al., entitled "Implantable Device With Microprocessor Control," represents one example of a prior art pacemaker which takes advantage of the processing speed of a microprocessor as compared to the length of a human cardiac cycle. In the Gordon et al. '972 reference, a pacemaker is described in which various electronic subsystems, including the pacemaker's control microprocessor, are fully activated only in response to predetermined "wake-up" events. For example, when the microprocessor completes the operating routine specified to occur following the occurrence of one event (e.g., a ventricular event), one or more wake-up events (e.g., an atrial event) is identified, and the microprocessor is deactivated. Circuitry for detecting the desired wake-up event(s) remains activated, so that upon occurrence of a wake-up event, the microprocessor and other appropriate electronic subsystems can be re-activated. However, since the microprocessor is deactivated for much of the interval between the two events, overall power consumption is reduced.
In U.S. Pat. No. 5,350,407 to McClure et al., entitled "Implantable Stimulator Having Quiescent and Active Modes of Operation," there is proposed an implantable pacemaker system in which the pulse generator is selectively operable in either a quiescent state or an active state. After manufacture of the device, the device is placed into its quiescent state, in which various sub-circuits are disabled, thereby reducing the device's power consumption prior to implant. A "wake-up" circuit continues to be operable, so that a clinician can subsequently fully reactivate the device, for example, with an activating command transmitted from an external programming unit. This arrangement is characterized as minimizing device consumption particularly during the "shelf-life" of the device, i.e., the period of time between manufacture and actual implantation of the device.
In U.S. Pat. No. 5,476,485 to Weinberg et al., entitled "Automatic Implantable Pulse Generator," there is proposed another pacemaker system that is provided with capabilities intended to minimize power consumption during the "shelf-life" of the device. The Weinberg '485 reference appears to propose a pacemaker having the capability of automatically detecting when the device has been implanted, such that prior to such detection the pacemaker can operate in a reduced power consumption mode. The implant detection is based on an assessment of lead impedance, and may also be based on an assessment of device temperature.
Notwithstanding the various measures that can be taken to minimize an implantable device's power consumption and hence maximize device longevity, battery depletion is inevitable, and pacemakers are often designed to have certain features which take this inevitability into account. For example, many pacemakers are provided with the ability to communicate an "elective replacement indicator" ("ERI") to an external programmer. The ERI informs the physician or clinician that the device's power supply is nearing, but has not yet reached end-of-life ("EOL"), the point at which the power supply cannot provide sufficient energy to keep the device operable. The advance warning provided by an ERI gives the physician the opportunity to take the appropriate measures, e.g., to replace the device, prior to EOL. Additionally, the device may automatically turn off various features processes or therapies to can serve power, responsive to the occurrence of an ERI situation. These operations can be critical for patients whose lives depend on operating their implantable devices.
It should be noted that many microprocessor patents have issued that provide many alternative power saving partial shut down and start up features, the teachings of which could be employed depending on the integrated circuit designers' needs for the implantable's microprocessor and its limitations, as well as the feature set of the implantable device.
Implantable devices also typically have an "end-of-life" or EOL function, which causes the device to enter into a special mode of operation when the device's power supply output falls below some predetermined threshold deemed to constitute the power supply's end-of-life. This special mode may be one that is less demanding on the power supply, so that at least basic functioning of the device can continue for some period of time. This may cause additional features to shut down beyond those which were shut down to conserve power for an ERI condition, or, if desired, this indicator (the EOL signal) may be the first to cause any power saving action.
Another feature commonly included in state-of-the-art implantable devices is a so-called "power-on reset" ("POR") function. POR functions typically involve placing the device into a special POR mode of operation whenever power is first applied to the device. That is, the device will begin operating in the POR mode when the power supply is first connected to the device (i.e., at the time of assembly), and at any later time when power is disrupted and then restored. The latter can occur for various reasons, including when the device is subjected to electrical noise or electro-cautery discharges, or when the battery temporarily becomes very cold, reducing the battery's output below that which is needed to maintain device operation. As will be seen in discussion later, various special functions can be automatically started responsive to a POR signal.
The cold-battery POR situation can be particularly problematic in terms of erroneously or unnecessarily putting the device into the POR mode. The most likely situation that will cause an implantable device to be triggered into its POR mode is exposure to a period of extremely low temperature while it is being shipped from the manufacturer to the customer, e.g., a doctor or hospital with a subsequent return to normal temperature. If this occurs, then the device will be operating in accordance with its POR mode (usually spare) settings, which are usually different than the preferred default settings that the manufacturer will program the device to prior to shipping to the customer. This is believed to be undesirable, as it requires the customer to (1) check every device upon receipt to determine whether it has been triggered into POR mode; and (2) reprogram those devices which have been triggered into POR mode. Additionally, POR or ERI indicators may be set or circuit timing or power settings changed due to a patient's ice-bath or sonic treatments for kidney stones, electrocautery, and being subjected to defibrillation pulses.